This invention relates to an apparatus for operating plural electric motors connected to a common load through respective connecting devices including gear wheels. More particularly, it relates to driving the load smoothly in such apparatus through controlling the excitation of the field winding of one of the electric motors which is a synchronous induction motor.
In the below, the conventional twin drive system for driving a cement mill is described by way of an example. FIG. 1 shows two synchronous induction motors being operated for driving the mill and FIG. 2 the circuit of an exciter for the synchronous induction motor. In FIG. 1, the numeral 1 designates a mill having gears, not shown, along its periphery. The numerals 2, 3 designate pinions meshing with the gears of the mill 1 and the numerals 4, 7 designate synchronous induction motors for driving the mill 1 through the associated gears. The outer periphery of the stator of motor 4 is provided with a gear, not shown, for meshing with a pinion 5. The numeral 6 designates a turning device having braking means, not shown and adapted for driving the pinion 5. The turning device 6, the pinion 5 and the gear provided to the stator of the motor 4 make up a stator shifting device for rotating the stator of the motor 4 a required angle. The numeral 8 designates a feeder or bus connected to a three-phase source which is connected to the stator of the motors 4 and 7.
Referring to FIG. 2, the numeral 10 designates a three-phase delta winding provided to the rotor of the motor 4 having phase windings 11, 12, 13 independent from one another. The numeral 9 designates a slip ring unit consisting of six slip rings. The numeral 20 designates a damper circuit connected to the R-phase winding 11. The numerals 21, 23 designate contactors and the numeral 22 designates a starting resistor. The numeral 24 designates a resistor of lower resistance. The numeral 30 designates an exciter circuit connected to the S-phase winding 12 and comprised of circuit elements 31 through 34 and the numeral 40 an exciter circuit connected to the T-phase winding 13 and comprised of circuit elements 41 through 44. These circuit elements 31 through 34 of the exciter circuit 30 and the elements 41 through 44 of the exciter circuit 40 correspond to the circuit elements 21 through 24 of the damper circuit 20 mentioned above. The circuits 30, 40 are connected to D.C. sources 35, 45, respectively. The rotor winding 10 is composed of the R-phase winding 11 acting as damper winding and S-phase as well as T-phase windings 12, 13 acting as exciter windings. The rotor winding and the exciter of the motor 7 are constructed similarly to those of the motor 4 depicted above.
The operation of the twin drive system is now described by referring to FIGS. 1 and 2. The stators of the synchronous induction motors 4, 7 are connected to the common source 8. In FIG. 2, the contactors 21, 31, 41 as shown are normally open and are closed for initiating the self-starting of the motor 4. The motor 4 is accelerated by adjusting the starting resistors 22, 32, 42. When the motor speed approaches to the synchronous speed, contactors 23, 33, 43 are closed, while contactors 21, 31, 41 are opened. As a result, current flows from the D.C. sources 35, 45 of the exciter circuits 30, 40 to the rotor windings 12, 13 so that the motor 4 enters into synchronous operation under load. At this time, the circuit 20 is acting as damper circuit. Similarly, the motor 7 enters into synchronous operation and the mill 1 is driven at a rated speed by motors 4, 7 through gears 2, 3.
Supposing that two synchronous induction motors 4, 7 of the same design or rating are connected to the same source and to the mill 1 under the same conditions to take care of the load, the internal phase angle or the angle between the center of the revolving field generated by the three-phase winding of the stator (armature) and the center of the magnetic field generated by the rotor winding (field winding) is same for the two motors. Since the center of the revolving field is the same for the two motors through fabrication, the center or the pole of the revolving magnetic field produced by the field winding of one motor is rotated at all times at the same relative angular position as that of the other motor. Thus the same load is shared by motors 4, 7 under the same conditions.
However, in effect, since the gears of the mill 1 do not mesh with pinions 2, 3 in the same state due to manufacture tolerance or difference in wear, the rotors of the motors 4, 7 are not rotated at the same relative angular positions at the time of starting or during running. Hence, the motors 4, 7 may be operated with different internal phase angles thus causing a difference in load sharing. Thus the load on one motor may be increased while that on the other motor decreased correspondingly. This may give rise not only to motor troubles but damage to the gears.
In order to avoid this, it is essential that the internal phase angles of the two motors be matched to each other. To this end, with the prior-art system shown in FIG. 1, the angular position of the stator of one motor 4 is shifted by a stator shifting device such as turning device 6 for shifting the center of the rotating magnetic field associated with the stator of the motor 4 in such a manner the internal phase angles of the motors 4, 7 may be matched to each other.
In the conventional operating system depicted above, the structure of the stator shifting device tends to be complex and expensive while requiring intricate maintenance. In addition, the motor stator tends to be large in size thus making it difficult to manufacture a motor capable of producing a large torque.
It should be noted that use of synchronous motors in the driving system of FIG. 1 for driving the mill 1 gives rise to a similar inconvenience and that, when pneumatic clutches are interposed between the pinions 2, 3 and the motors 4, 7, difficulties are presented in supplying compressed air to the pneumatic clutches of the motors 4, 7 under the same conditions. Thus, clutch operation is not synchronized accurately with the two motors, which may be rotated at different angular positions relative to each other.